[debug-early] Cache DW_TAG_label's if appropriate

[Aldy Hernandez]

I'm running the gdb testsuite with the debug-early branch compiler, and 
fixing regressions.

gdb.base/label.c was causing gdb to ICE because we were generating 
multiple DW_TAG_label's for the same DECL.  Consequently, I have fixed 
gen_label_die to handle multiple calls with the same DECL.

By the way Jason, is there any reason we previously called 
equate_decl_number_to_die() only for abstract instances?:

  if (DECL_ABSTRACT_P (decl))
    equate_decl_number_to_die (decl, lbl_die);
  else
    {
      insn = DECL_RTL_IF_SET (decl);

I was unable to find the original reasoning, because the original patch 
setting this was from the 1990s, and it came en-masse when dwarf2out.c 
was contributed.  I don't see anything else in dwarf2out.c doing a 
lookup_decl_die() on a LABEL_DECL, so I don't even know why the 
equate_decl_number_to_die() is even there.

Anyways, in the immortal words of Shaggy... "wasn't me", so I'm leaving 
this bit untouched.

Tested on x86-64 Linux with the gdb regression suite.

Committed to branch.
Aldy
commit 8fea2b5c37026b3121afc0a98630542bb9a733dd
Author: Aldy Hernandez <al...@redhat.com>
Date:   Wed Mar 18 09:55:38 2015 -0700

    Cache DW_TAG_label's if appropriate.

diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 30c6cc6..92f4903 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -19725,14 +19725,20 @@ static void
 gen_label_die (tree decl, dw_die_ref context_die)
 {
   tree origin = decl_ultimate_origin (decl);
-  dw_die_ref lbl_die = new_die (DW_TAG_label, context_die, decl);
+  dw_die_ref lbl_die = lookup_decl_die (decl);
   rtx insn;
   char label[MAX_ARTIFICIAL_LABEL_BYTES];
 
-  if (origin != NULL)
-    add_abstract_origin_attribute (lbl_die, origin);
-  else
-    add_name_and_src_coords_attributes (lbl_die, decl);
+  if (!lbl_die)
+    {
+      lbl_die = new_die (DW_TAG_label, context_die, decl);
+      equate_decl_number_to_die (decl, lbl_die);
+
+      if (origin != NULL)
+       add_abstract_origin_attribute (lbl_die, origin);
+      else
+       add_name_and_src_coords_attributes (lbl_die, decl);
+    }
 
   if (DECL_ABSTRACT_P (decl))
     equate_decl_number_to_die (decl, lbl_die);